And the United Kingdom's decision to leave the European Union - Brexit, as it's known - is expected to have a big impact. It may hit hardest along the border between Ireland and Northern Ireland, which is part of the U.K. Right now trade flows smoothly over that border. But after Brexit, business owners fear new tariffs. NPR's Frank Langfitt reports from Northern Ireland.

RONAN MCELHOLM: OK. Well, this is our workshop.

FRANK LANGFITT, BYLINE: Ronan McElholm owns ACS Controls, which makes control panels. There are big boxes with wires and switches that manage the heating and ventilation systems in commercial buildings.

MCELHOLM: This one here is just in the process of being tested at the moment. All our equipment is tested before it leaves the workshop.

LANGFITT: McElholm started the company in his garage and now sells about 200 control panels a year. He's optimistic about the business, except for one thing.

MCELHOLM: The Brexit for us is a disaster because it's just going to cause some real problems.

LANGFITT: The big problem is this. McElholm's workshop is in Northern Ireland. But most of his customers are across the border in the south in the Republic of Ireland. McElholm worries that after Brexit, he'll get slapped with tariffs.

MCELHOLM: We don't know if it's 10, 15, 20 percent that we'd be paying on each one of these units going out.

LANGFITT: Right now do you pay any tariffs or duties?

MCELHOLM: No, there's no tariffs or duties because it's all within the EU.

LANGFITT: McElholm already making contingency plans.

MCELHOLM: What I may have to do in worst-case scenario is split the company in two and have half of it here in Northern Ireland and another half in Donegal just to overcome that tariff.

LANGFITT: Is that efficient?

MCELHOLM: No (laughter).

LANGFITT: Manufacturers aren't the only ones worried. Farmers are, too. David Crockett tildes land on both sides of the border. He fears Brexit could force him to pay a tariff every time he wants to move his machinery from one side to the other. And Crockett thinks that authorities will seal many of the small roads that cross the border to prevent smuggling.

DAVID CROCKETT: This road here will close again.

LANGFITT: You think it'll close?

CROCKETT: Oh, yeah.

LANGFITT: Why?

CROCKETT: They'll say to me, you have to keep your machinery on both sides of the border, right? Are they going to have a customs man stand here and a customs man down there stopping every vehicle going across? It couldn't be done.

LANGFITT: Crockett says he can't afford to buy two sets of farm machinery.

CROCKETT: The way farming is at the present time, there isn't money enough for me to start to buy all machinery over there again for here. If I had to do that, I'd be out of business.

(SOUNDBITE OF WATER FLOWING)

GERRY TEMPLE: My name's Gerry Temple. And I used to be a customs officer.

LANGFITT: Temple's standing on another road along the border which follows the creek. He says smuggling was a way of life during the Troubles, the civil conflict between Catholics and Protestants that ran for three decades. Authorities tried to stop it then.

TEMPLE: The army put on large, concrete bollards. And the locals literally manhandled them out of the way. There was places where the army put on concrete constructions. And locals built bridges over them.

LANGFITT: And, Temple says, officials won't be able to stop smuggling after Brexit because there's simply too many border crossings.

TEMPLE: It's mission impossible. If they think for one moment that they can monitor 350-odd roads - it's just not doable.

LANGFITT: Ireland says the number is closer to 260, but you get the point. So does Simon Coveney. He's Ireland's foreign minister who oversees the country's policy towards Brexit.

SIMON COVENEY: There was a time a number of months ago when people were talking about solving these problems with technology, with scanning systems, with cameras. I have been very clear that I do not think that can work. And instead, what's needed here is a political solution to the border, whereby we effectively keep Northern Ireland as part of an extended customs union.

LANGFITT: What Coveney means is Northern Ireland would continue to adhere to the EU customs standards, negating the need for border inspections.

COVENEY: The problem with that, of course, is that it creates potential barriers to trade between Northern Ireland and mainland Britain.

LANGFITT: Which are part of the same country, the United Kingdom. Fix one problem. Create another. The future of the border remains uncertain. But Coveney says at least one thing has become clear since last year's Brexit vote.

COVENEY: More and more British people, I think, are beginning to understand that leaving the European Union is not as straightforward as they thought it would be.